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Download Article (PDF) American Mineralogist, Volume 80, pages 1031-1040, 1995 Zirconosilicate phase relations in the Strange Lake (Lac Brisson) pluton, Quebec-Labrador, Canada STEFANO SALVI,* ANTHONY E. WILLIAMS-JONES Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada ABSTRACT Petrographic observations of subsolvus granites at Strange Lake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions, but that the calcium zir- conosilicates armstrongite and gittinsite are secondary. This interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite. Both calcium zirconosilicate minerals show textural evidence of having replaced elpidite, and in the case of gittinsite, with major volume loss. In the near-surface environment, gittinsite plus quartz fill the volume formerly occupied by el- pidite. At greater depth, gittinsite and armstrongite partially replaced elpidite but are not accompanied by quartz, and abundant pore space is observed where gittinsite is the prin- cipal secondary phase. Below 70 m elpidite is generally unaltered. Replacement of elpidite by armstrongite is interpreted to have been a result of the cation-exchange reaction NazZrSi601s'3HzO + Ca2+ = CaZrSi601s.3HzO + 2Na+ elpidite armstrongite in which volume is nearly conserved, and replacement by gittinsite is thought to have resulted from the reaction NazZrSi601s.3HzO + Ca2+ + 5HzO = CaZrSiz07 + 2Na+ + 4H4SiO~ elpidite gittinsite which is accompanied by a 65% volume reduction. An alteration model is proposed in which external Ca-rich, quartz-undersaturated fluids dissolved elpidite and replaced it with gittinsite, where aH4SiO~was buffered mainly by the fluid (high water-rock ratio), and with armstrongite plus gittinsite, or armstrongite alone, where aH4SiO~was buffered to higher values by the rock (low water-rock ratio). The for- mation of gittinsite created extensive pore space that was subsequently filled, in the upper part of the pluton, when the fluid became saturated with quartz as its temperature de- creased during the final stages of alteration. INTRODUCTION curs as one or more of a large family of alkali and al- In most igneous rocks, Zr is an incompatible trace kaline-earth zirconosilicate minerals. The most impor- element present in amounts ranging from < 10 to several tant of these are catapleiite (NazZrSi309. 2HzO), dalyite hundred parts per million and is accommodated as the (KzZrSi601s), elpidite (NazZrSi601s' 3H20), eudialyte accessory mineral zircon. However, in peralkaline ig- [Na4(Ca,Ce)z{Fe2+ ,Mn,Y)ZrSig022(OH,Cl)zJ, vlasovite neous rocks its concentration is commonly orders of (NazZrSi4011), and wadeite (KzZrSi309), any of which magnitude higher and may be locally sufficient to con- may be the principal zirconosilicate mineral in a partic- stitute a potentially exploitable deposit (e.g., Ilimaussaq: ular setting, e.g., eudialyte at Lovozero (Vlasov et aI., Gerasimovsky, 1969; Lovozero: Kogarko, 1990; Strange 1966), catapleiite at Mont-Saint-Hilaire (Horvath and Lake: Miller, 1986; Kipawa: Allan, 1992; and Thor Lake: Gault, 1990), and vlasovite on Ascension Island (Fleet Trueman et aI., 1988). The Zr mineralogy of peralkaline and Cann, 1967). rocks is also unusual. Although most peralkaline rocks Little is known about the factors controlling the distri- contain some zircon, the bulk of the Zr commonly oc- bution of zirconosilicate minerals, and there is little doc- umentation of their parageneses. The only reversed ex- perimental study that provides data on their stability at ... Presentaddress:Laboratoirede Geochimie-CNRS,Unive!"- geologically meaningful conditions is that of Currie and site Paul Sabatier, 38 rue des Trente-Six Ponts, Toulouse F-31400, Zaleski (1985) on the dehydration reaction of elpidite to France. vlasovite and quartz. Marr and Wood (1992) constructed 0003-004X/94/091 0-1 031 $02.00 1031 1032 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS ++++n.r+ + + + +J1L ~ + + + + + + + + ~ + + + + + + + + + . + + + + + + *+ + + APHEBIAN BASEMENT amphibolite and metagabbro metadiorite quartzo-feldspathic & graphitic gneiss calc-silicate gneiss Fig. 1. Location and simplified geological map of the Strange Lake pluton (modified after Salvi and Williams-Jones, 1992). theoretical P-T diagrams for sodium and calcium zir- the calcium zirconosilicate armstrongite, which they conosilicate minerals using the results of Currie and Za- claimed occurs both as phenocrysts and as a replacement leski (1985), molar volume data, and the few constraints of elpidite. available from natural systems. Further advances in our We reexamine the genesis of elpidite, armstrongite, and understanding of Zr mineral petrogenesis will require ba- gittinsite at Strange Lake, in light of textural and mineral sic thermodynamic data for the major phases and careful chemical data collected from a representative diamond- documentation of field occurrences. The latter is the ob- drill hole located near the zone of economic interest. These jective of the present contribution. data, in conjunction with mineral stability relationships In the Strange Lake pluton, Zr occurs mainly in the and published fluid-inclusion data (Salvi and Williams- form of the zirconosilicate minerals elpidite, armstrongite Jones, 1990), support a model involving low-temperature (CaZrSi6015.3HzO), and gittinsite (CaZrSiz07). These metasomatic replacement of elpidite by armstrongite and minerals vary greatly in their relative proportions, and, gittinsite. locally, anyone of them may be the principal or sole Zr phase. The highest concentrations of Zr are in rocks con- GEOLOGICAL SETTING taining only gittinsite. Salvi and Williams-Jones (1990) The Strange Lake pluton (1189 :t 32 Ma, Pillet et aI., observed that gittinsite plus quartz form pseudomorphs 1989), located in the eastern Rae province of the Cana- after an unidentified phase; on the basis of the morphol- dian Shield, is a peralkaline granite that intruded Aphe- ogy of the pseudomorphs and on fluid-inclusion evi- bian metamorphic rocks (1.8 Ga) and an Elsonian quartz dence, they suggested that a Ca-bearing fluid at -200°C monzonite (1.4 Ga) (Fig. 1). The intrusion is a subverti- caused the metasomatic transformation of the sodium cal, cylindrical body approximately 6 km in diameter and zirconosilicate elpidite to gittinsite + quartz. Birkett et represents the latest tectonic event in the area. Large roof aI. (1992) subsequently proposed that gittinsite replaced pendants are commonly observed (Fig. 1), suggesting that SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS 1033 the present erosional surface is close to the apical part of the pluton. On the basis of its peralkalinity, mode of em- placement, and age, the pluton may represent the Lab- rador extension of the Gardar anorogenic igneous event (Currie, 1985; Pillet et ai., 1989). 18.0 Several concentric intrusive pulses ofpera1kaline gran- ite formed the Strange Lake pluton, represented by hy- persolvus granite, transsolvus granite, and more evolved, 16.0 volatile-saturated subsolvus granites (Nassif and Martin, 1991) (Fig. 1). All phases are F-rich and contain primary fluorite. They are mainly leucocratic, fine to medium 14.0 grained, and quartz saturated. Pegmatites are common and concentrated mainly in the subsolvus granite. An outwardly dipping ring fracture, marked by a heteroge- J.. 12.0 neous breccia in a fluorite-filled matrix, surrounds the N pluton. E The hypersolvus granite consists largely of phenocrysts 0. 10.0 of quartz and perthite, and interstitial arfvedsonite, 0. whereas the subso1vus granite contains two alkali feld- 0 0 8.0 spars (albite and microcline), and arfvedsonite commonly 0 V occurs as phenocrysts. Transsolvus granite is transitional or- LO to the two other types of granite, i.e., it contains perthite ~-II >< phenocrysts in addition to albite and microcline. Arfved- 6.0 II c::: sonite in this unit occurs predominantly as fine-grained c::: anhedra, imparting a melanocratic appearance to the rock. The hypersolvus granite contains up to 5% of zirconosil- 4.0 icate minerals, and the subsolvus granite and pegmatites contain up to 30% of these minerals. Elpidite is the prin- cipal Zr mineral in the hypersolvus granite, whereas el- 2.0 pidite, armstrongite, and gittinsite are the dominant Zr minerals in the subsolvus granite and pegmatites. Less common zirconosilicates at Strange Lake include cata- 0.0 p1eiite, calcium catapleiite, dalyite, hilairite (Na2ZrSi309. 3H20), vlasovite, and zircon (cf. Birkett et ai., 1992). Subsolvus granites commonly display hematitic alter- ation and, where altered, have higher Zr contents than fresh hypersolvus or subsolvus granites (Fig. 2). Hema- Fig. 2. A histogram showing the average whole-rock content tization was associated with the development of second- of Zr in the principal varieties of granite at Strange Lake. Ab- ary high-field-strength element (HFSE) minerals, many breviations: hs = hypersolvus, ts = transsolvus, f-ss = fresh sub- solvus, a-ss = strongly altered sub- of which are Ca-bearing. Some of the most intense he- altered subsolvus, and s-ss = solvus. Standard deviations are 1.9,2.4, 1.3,2.3, and 0.3 x 1000 matization is in the center of the complex, where a sub- ppm, respectively. economic ore zone has been delineated containing some 30 million tonnes grading 3.25% Zr02, 1.3% REE oxides, 0.66% Y203, 0.56% Nb20s, and 0.12% BeO (Iron Ore
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